1. Field of the Invention
This invention relates to lasers, and more particularly to the use of a certain class of dyes as lasing media for dye lasers and laser devices.
2. Description of the Background
Many of the materials discovered thus far which are capable of acting as lasing media have been in the solid or gaseous state. Solid lasers typically suffer from such disadvantages as cracking and optical imperfections. It is also known that certain organic dyes in solution can operate as "liquid" or "organic dye" lasers. Of the range of materials useful as lasing media, organic lasing dyes have certain advantages. A wide range of organic dye lasers is available to provide stimulated emission (lasing) over a broad range of the spectrum. In addition, organic dye lasers are generally capable of being tuned to emit over a range of wavelengths, this being in contradistinction to the essentially single wavelength capability of lasing emission characterizing gas and solid lasers. Moreover, organic dye lasers provide an economical lasing medium when compared to gas and many solid lasers, and they do not suffer from disadvantages such as cracking and optical imperfections that are particularly associated with solid lasers.
The ability to selectively tune organic dye lasers derives from the broad band fluorescence characteristic of the component dye. Such lasers can be "tuned" to emit at wavelengths substantially along the entire fluorescence band of the dye by interposing a dispersive element such as a diffraction grating or a prism.
The operation of a laser is achieved as a result of the phenomenon that excited atoms or molecules can emit a photon or quantum of light. This photon or quantum can itself trigger another excited atom or molecule to emit its photon prematurely. This process is called designated stimulated emission.
The excitation of organic lasing dyes can be achieved by subjecting the dye, under controlled conditions such as will be described herein, to a suitable source of energy. One example of a source of energy is bombarding with electrons or illuminating with a high energy source. Conventionally, illumination is utilized for liquid laser materials. Excitation of a liquid laser medium by illumination is generally termed "optical pumping" or merely "pumping." Pumping sources include, for example, sources such as giant pulse lasers, xenon and argon arc flash tubes, as well as arc discharge tubes containing only air or other gaseous mixtures. Similar laser action is expected in a solid polymer medium when it is pumped optically and being contained in a laser cavity.
Various arrangements of laser apparatus can be used. A laser structure particularly adapted for organic dye liquid laser media is described by Sorokin, et al., IBM Journal II:148 (1967). Advantageous laser apparatus structures usually include an optically resonant cavity containing a reservoir of a liquid laser medium or a liquid laser body disposed within a thin-walled quartz cylinder. Typically, the reservoir is part of a closed system through which the dye solution is circulated during the lasing operation. Thus, localized heating which can cause refractive discontinuities and potential breakdown of the dye is effectively prevented. To provide an energy source for exciting the atoms of the laser material, the laser body may be surrounded concentrically by a lamp, such as one containing an annular region within an outer thick-walled quartz cylinder. The annular region may contain an air-argon mixture and have electrodes which are operably connected to a low inductance capacitor charged by a standard high voltage supply. Desirably, coaxially disposed at either end of the optically resonant cavity are opposed internally reflective cavity ends such as mirrors. When optical pumping is used, the light source emits light having wavelengths within at least one absorption band of the lasing medium. The absorbed light causes molecular electrons in the medium to shift in energy level. Molecular electrons exist either in a singlet state (two electrons spinning in opposite directions) or a triplet state (two electrons spinning in the same direction). The ground state is the unexcited state for molecular electrons and has the lowest energy. Typically, the ground state in almost all molecules is a singlet (designated S.sup.0), one of many possible energy levels in the singlet state. When the pumping source is activated, the resultant light pulse enters the laser body and photons of energy of appropriate absorptive wavelength are absorbed by active molecules in the body and cause the electrons of such molecules to shift from an initial low energy level (S.sup.0) to a high energy level from which emissive transition occurs.
In operation, the molecular electrons of the laser medium are desirably "pumped" to higher excited states of the singlet system by intense energy inputs. It is thought that they then first undergo transitions from such excited states to the lowest excited state (designated S.sup.1). After diminishing in energy level to the lowest excited singlet, the molecule can relinquish its remaining excess energy radiatively or non-radiatively from S.sup.1 to S.sup.0, non-radiatively from S.sup.1 to a triplet state and then radiatively or non-radiatively from the lowest excited triplet state to S.sup.0. Generally, laser emission consists of optical emission resulting from transitions from S.sup.1 to various vibrational modes of S.sup.0. Susceptibility to triplet formation upon pumping is deleterious due to typical non-radiative energy losses resulting from triplet to S.sup.0 transitions. Moreover, if there is significant overlap between the triplet absorption and either the pump bands or lasing emission bands, laser action generally will be impeded or will fail entirely. Additionally, advantageous laser emission can occur only when the population of molecules established at this higher energy level in the laser body by such light pumping exceeds the population of molecules remaining at the initial low energy level, a condition conventionally designated "population inversion" or "inversion of energy states."
Upon reaching an inversion of energy states, individual molecules of the high energy level population undergo emissive transition spontaneously, shifting to a terminal low energy level as described herein with a concomitant emission of light. A portion of the spontaneously emitted light is usually reflected back and forth through a resonant optical cavity structure between its internally reflective ends. As this light passes through the laser body in multiple bidirectional reflections, it induces other molecules of the enlarged high energy level population to undergo premature light emissive transitions as noted herein. This produces more light, which augments the bidirectionally reflected light in the cavity to induce still further light emmissive transitions. A rising pulse of bidirectionally reflective light quickly develops in the cavity reaching a quantitatively large value as the induced emissive transition of molecules from the high energy level population increases. If one of the reflective cavity ends is partially transmissive, as is typically the case, a portion of the intense reflected light pulse passes through the one end and out of the cavity to constitute the laser output light pulse or the laser beam.
As mentioned previously, organic dye lasers have many advantages over solid and gas lasers. However, depending upon the wavelength of lasing desired, one must choose entirely different classes of dyes to obtain the appropriate result.
Conducting polymers were discovered in the late 1970s, suggesting the possibility of combining the important electronic and optical properties of semiconductors and metals with the attractive mechanical properties and processing advantages of polymers. Initial efforts to this end were discouraging since the new conducting polymers exhibited several undesirable properties including among others insolubility, intractability, relatively poor mechanical properties and moreover such polymers were non-melting.
More recently, specific conjugated polymers systems have been rendered more soluble and processable. For example, the poly(3-alkylthiophene) derivatives (P3ATs) of polythiophene are soluble and meltable with alkyl chains of sufficient length, and the P3ATs have been processed into films and fibers. However, due to the moderate molecular weights and/or the molecular structures of these polymers, the mechanical properties, particularly the modulus and tensile strength of fibers and films made from these polymers are insufficient to enable their use in many applications.
Alternative methods of processing conductive polymers have been developed. For example, poly(phenylenevinylene), PPV, and the alkoxy derivatives of PPV are synthesized via the precursor polymer route.
Other efforts have been directed to the development of p-polyphenylenevinylene, PPV, polymers which are soluble in the final conjugated form. Poly(dihexyloxy phenylenevinylene), DHO-PPV, is not soluble in common organic solvents at room temperature, but is soluble at temperatures above 80.degree. C. The longer side-chain octyloxy derivative, DOO-PPV, was found to be less soluble in most non-polar solvents, probably because of side chain interdigitation and "side chain crystallization".
Materials available for use as chromophores in dye lasers have been small conjugated organic molecules, used either in solution or in blends with various host materials such as amorphous polymers, sol-gel glasses, etc. For example, coumarin dyes such as umbelliferone are useful in the short wavelength region, fluorescein is useful in the medium wavelength region, and rhodamines, such as Rhodamine 6G, are useful in the longer wavelength region, as are long chain cyanine dyes, such as 3,3'-diethylthiatricarbocyanine iodide.
Recently, a new class of conducting (conjugated) polymers has emerged which offers opportunities for applications in photonics. Within this class, poly-phenylenevinylene and its derivatives are particularly promising, for they exhibit both photoluminesence and electroluminescence. Light emitting Schottky diodes made of (2-methoxy, 5-(2'-ethyl-hexyloxy-p-phenylenevinylene), MEH-PPV, in contact with calcium have been demonstrated with quantum efficiency of approximately 1% and with good brightness. The relatively high quantum yield for photoluminescence suggests the possibility of achieving polymer lasers in various forms: for example, dye lasers using the conjugated polymer as the active chromophore either in solution or in blends with another host polymer or any other suitable host material, as well as solid polymer diode lasers. This technology was disclosed in U.S. application Ser. No. 07/635,455 entitled PROCESS FOR FORMING POLYMERS, filed on Dec. 17, 1990 by Wudl. et al.
From a synthesis stand point, it can be readily appreciated that generally it is less expensive to manufacture various members of a single class of dyes than to manufacture several distinct classes of dyes in order to obtain the desired wavelengths of lasing.
Accordingly, there is a need in the art for a class of conductive dye laser and laser devices having superior lasing characteristics without the disadvantages and draw backs of the prior art.